The determination of the thermal effect accompanying the change of one allotropic modification of an element into another is based on this principle. The heat of combustion of a certain red phosphorus is less by 20*7 Cal. than that of the yellow modification : the transformation of this latter into the red variety is therefore accompanied by an evolu-
The heat of hydration of acids, of bases, and of salts, the difference between the heats of neutralisation of two different acids by the same base, or of two different bases by the same acid, and the heat of formation of organic compounds are further examples of this same indirect method. For details the reader is referred to the following chapters.
Solution
Gases which dissolve in water according to Henry's law have a very low heat of solution.
Amongst those gases which do not obey this law, there are some whose solution is attended with a very con- siderable heat effect, greater even than the heat of liquefaction of the gaseous substance. The difference between these two heats represents the heat evolved by the action of water on the liquefied gas.
Examples (Berthelot) :
Heat of Solution Heat of Liquefaction
(per gram-molecule) (per gram-molecule)
Chlorine ... 30 Cal.
Bromine . . 8*3 „ 7*0 Cal.
Hydrochloric acid 17*4 „
Hydrobromic „ . 20*0 „
Hydriodic acid . . 19*6 „
Ammonia . . . 8*8 „ 4-4 Cal.
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��Carbon dioxide . . 5*6 „ 2*5 ,, at 0°.
��Sulphur dioxide . . 8*3 „ 6 ,, (about).
The solution was made in much water (100 to 500 molecules) 15°C.
��1 Bed phosphorus exists in several states which require further investigation.
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